Dynamic Paint - "Use Object material" for Brush removed in Blender 2.8?
In the end I didn't use the material color, but used a small script to assign the vertex color of each object to the brush color of a dynmaic paint modifier. (Use Strg+L to link modifier to all objects)
for obj in bpy.context.collection: #use active collection
    obj_col = list(obj.data.vertex_colors['Col'].data[0].color)[:3]   
    obj.modifiers['Dynamic Paint'].brush_settings.paint_color = obj_col